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Tuesday, 15 September 2020

ammoon 5.8G Guitar Wireless System

 Wires can be such a nuisance when you are playing an instrument, it is easy to tread on the wire and pull the plug out - or even worse break the cable or plug. Also other people can trip over them. I wanted to get a wireless link for my guitar, and I also wondered if I could use it as an audio link between a mixing desk and a PA amplifier.

After looking at what was on offer on Ebay and some of the music shops I decided to buy this:

It has a very good spec. 10Hz to 20kHz, 110dB dynamic range, 24bit digital audio. Some of these devices work on 2.4GHz and are a bit cheaper, but this one is on 5.8GHz, where I figured there would be less interference from Bluetooth, WiFi, microwave ovens, etc. Also the 2.4GHz version has a slightly less good dynamic range.

So here's my review. 

First thing was to use a USB adaptor to charge the two devices up. It comes with a special "Y" USB cable, so both can be charged from on charger. Each one takes 300mA, so 600mA for the pair. The batteries are said to be 600mAh, so it should charge up in 2 hours, from completely flat. 

The LED's don't line up with the windows in the case, which is a bit annoying, but you can see that the RED charge LED is on by turning it slightly sideways. It is the same with the green LED which indicates whether the two units are "paired" correctly. 

Once fully charged, I plugged one end into my Tanglewood electro-acoustic guitar and the other end into a combo amp, and straight away it worked. I didn't have to mess about pairing the devices, just switch them on. Nice sound too. Turning the EQ and the volume to maximum on the guitar and playing loudly - no distortion. It is difficult to tell how noisy it is because the amp is quite noisy anyway, but it didn't seem to add any extra noise.

One thing the specification doesn't tell you is what the maximum signal handling is. So I set it up on the bench, with the signal generator and an oscilloscope. 

I found that I could turn the input signal (at 1kHz) up to 2V peak to peak (0.7V r.m.s.) where clipping just started to occur. It didn't do anything nasty with the onset of clipping, the top of the waveform was cut off in the usual way.

The clipping level was the same at all frequencies as far as I could tell. 

In the spec it says that the delay is less than 6mS, I measured it as 6.2 mS, which is close. The signal at 10Hz is inverted, but the effect of the delay is that the signal is back in phase at 750Hz and then goes in and out of phase with the input as the frequency is increased. So be aware that strange things will happen if you mix the input signal with the output.

One thing that did worry me a bit, was the fuzzy trace (see photo at left. There is about 20mV of noise superimposed on the output signal. 
The oscilloscope here is set to 50mV/div with a 1kHz signal input. 2mS/div on the horisontal axis. The top trace is the output of the receiver, and the bottom trace is the input from the signal generator. You can also see the phase shift here, and also that there is a very slight gain through the system. The output is about 12% bigger than the input.

When I looked at the noise with the input signal turned off, I couldn't trigger on it, and I think it is all very high frequency. It maybe some kind of breakthrough of the sampling frequency. 
I might add a passive filter to remove it when I use it with the PA amplifier, just to preserve the tweeters!

All considered I am quite happy with this purchase. seems good to me.

The signal


Tuesday, 21 July 2020

6dB Attenuator

Sometimes it is useful to be able to reduce the output of a radio transmitter below the lowest power setting on the radio. This was the case when I wanted to drive a 23cms transverter from a 2m multimode radio. Technically the 2.5W output of the FT290 was within the input range of the transverter, but I figure that all the unused power has to go somewhere, and I didn't really want it heating up the transverter.

So the requirement was for a reduction of power by about a factor of 4, which is 6dB in decibels. But it would also need to handle 2.5 Watts, safely.  The input and output impedance needed to match the 50 ohm system impedance.
There is a tutorial for designing pi attenuators here:
Which even gives the values for a 50 ohm 6dB version in the table. 150.5 ohms and 37.4 ohms.
I checked out the design in LTSpice
37.4 ohms is not readily available, however, I worked out that four 150 ohm resistors in parallel gives 37.5 ohms. Paralleling resistors also allows them to handle higher powers. Wire wound resistors will not work at VHF, because they behave as inductors. Surface mount (SMT) film resistors would be good. I found that the 2012 size SMT resistors can handle 750mW each so in theory four should cope with 3 Watts, although some derating is needed if they are close to other resistors that are generating heat. It is also possible to combine four 150 ohm resistors, using two series resistors in a parallel pair combination, to give 150 ohms. Using LTSpice, I was able to measure the current in each resistor and calculate the power dissipated. In fact R3 and R6 share most of the power, equally between them so with good heat-sinking, the required 2.5W rating will be comfortably exceeded.

Then I did something I haven't done for years. I etched a printed circuit board using ferric chloride. I cleaned up a piece of board and cut it to fit in the dicast aluminium box. Then I masked the areas of copper that I wanted to keep using PVC electrical tape, and schmoggled it about in ferric chloride for about 10 minutes, until the exposed copper had gone. Gave it a good wash and ended up with what you see at left.

On a double-sided PCB, it is possible to give the tracks a characteristic impedance, like coax cable. If this is made to be the same as the system impedance then it will help to ensure a good VSWR at the input. Using a table in the back of the VHF/UHF Manual (Edited by G.R. Jessop, G6JP, 4th edition, published by the RSGB in 1983), I made the tracks approx 3mm wide - although it is not possible to do this where the resistors are in parallel, I wrapped copper tape around the edges, so that the ground was well connected to the copper underneath.

When I came to mount the board in the box I cut three aluminium plates from 2mm thick sheet, so that the board was sitting on a 6mm thick block of aluminium. This brought the board up to a level where the centre pin of the BNC connector could be soldered to the PCB, using a very short wire. Again this helps keep the VSWR low, because lengths of wire act as inductors. I used a smear of heat sink compound between each of the plates, because I wanted the heat from PCB to be conducted to the outside of the box.

Before I finally assembled everything, I tested the board at d.c., measuring the resistance at input and output. Remember that you need to terminate the attenuator with 50 ohms if you want it to measure 50 at the input. When the output is open circuit it measures about 83 ohms at the input.

I used a bench power-supply to feed 11.2V (equivalent to 2.5W RMS) into the attenuator, and left it for about 20 minutes. Without the aluminium block, the board does get quite warm, almost too hot to touch, so I guess it is running about 60 degrees C. The aluminium helps to draw some of the heat away, so it runs a bit cooler when it is in the box.
With the lid screwed down, I fed some RF into it, and measured the VSWR. At 145MHz, the VSWR is very low. There is about 0.1W of reflected power with about 3W forward power. Pleasingly, it has a similar low VSWR at 433.000 MHz too - which is always a good sign.

Hugh M0WYE

Monday, 13 July 2020

Walk along the Byways and Ridgeways

Fantastic walk out to Crundale Church, up the ridge to Pett Lane Farm, and back over the Downs to Wye.



Saturday, 4 July 2020

MFJ-949E Capacitance and Inductance Settings

Like many amateur radio operators, I use an "Antenna Tuner" to create a good match between my HF transceiver and the feeder and antenna. Really, the best place for a matching device is at the terminals of the antenna, because this ensures a low VSWR on the feeder cable, and not just the output of the transmitter.
However, most of us find it much more convenient to have the tuner alongside the radio, and tolerate the additional loss which having a high VSWR on the feeder causes.

The tuner that I use is a manual one, an MFJ-949E. I have had it for many years and it seems reliable and well made. It has a number of useful features such as a built-in VSWR meter, and a dummy load, but the part which creates the good match between the radio and the antenna system consists of just three components. MFJ provide a circuit diagram in the back of the manual.

The circuit is a "T" filter, with two variable capacitors and a switched inductor. The design has varied a little bit over the years, and I have seen an older version which had a kind of sliding wiper on the inductor to vary its value.

The MFJ circuit diagram shows C1 and C2 are 208pf, but no values are given for the inductor. I thought it would be nice to measure the values of the capacitors and inductors at each position, because this would allow for some analysis of the antenna impedance. The impedance of the matching device should be the "complex conjugate" of the antenna impedance. Which basically means the reactance will be equal and opposite. But the situation is made more complex as the length of the feed line will transform the values. A 1/4 wave line will make the values opposite, and after a half-wave they will be the same again, repeating along the length of the line. If you want to know more about this look up the "Smith Chart" which allows this transforming effect to be calculated easily.

Transmitter / Antenna Control
Capacitance Value (pF)
Inductor Control Position
Inductance Value (μH)

The Transmitter and Antenna capacitors C1 and C2 appear to be identical. The controls are marked 0 to 10, but lining up the dot on the knob with the number is rather approximate.

The table shows that the capacitors have a linear law, and that there is about 20pF between each step from 220 down to 20pF. The inductor is a bit more "logarithmic" in its steps, but the switch makes the setting precise and repeatable.
I find it counter-intuitive that the capacitor numbers and inductor letters get larger as the values get smaller.

I measured these values on a DER EE,  LCR meter DE-5000, with the measurement frequency set to 100kHz.

Hugh M0WYE

Tuesday, 9 June 2020

Throat Microphone for bicycle mobile

I wanted to try using a throat microphone while operating the radio mobile on a bicycle. The big problem is wind noise on any conventional mic, but a throat mic is a contact microphone which picks up the vibrations directly through the neck.
Apparently they are often used by security guards and the like for discreet radio communications because they can be hidden under a collar.
I bought one of these.

The two-pin jack plug connects to my TYT MD380 DMR radio, so it was quite easy to try out. The Ashford repeater GB7AS features an echo facility where you can talk for a few seconds, and then the repeater plays back a recording of your transmission. I found that the audio was quite muffled, but was best when I moved the microphone as high up the neck as possible.

But I also noticed another problem. On high power, there was a "puttering" noise on the transmission, which seems to be the radio frequency signal getting into the microphone and causing interference. The noise went away on low power, but was very noticeable on high power, where-ever I positioned the radio antenna.

Undaunted, I took the equipment for a 12 mile cycle ride up onto the North Downs. I was unable to raise anyone on the Charing repeater GB3CK (using analogue transmission), but I did manage to get a signal report from Peter GW6YMS on Anglesea in Wales on DMR, via GB7AS.
"Unintelligible"  was his verdict, and "there's terrible motor-boating on your transmission" - which didn't go away even when I switched to low power. I gave up and talked to him using the microphone in the radio.

The earpiece part of the device does work quite well, although it tends to fall out of my ear after a mile or two. It is quite clear to listen to, private and comfortable to use - and I can still hear traffic coming up behind quite well too.

Curiosity got the better of me and I opened up the microphone and the little "junction box" in the cable, to see what was inside.
 You can see that the "microphone" is just a very small "piezo bender". This seems to be connected directly to the microphone input of the radio. There is a soft pad of foam behind the bender, and a small piece of blank fibreglass PCB. The wiring looks pretty horrible, with joints insulated by little dobs of hot-melt glue.

The junction box is equally ugly inside with more blobby joints and hot-melt glue.

It was very inexpensive.

I might try to make a boom microphone like I use in the car, with an electret capsule in a wind shield of some kind. I could keep the earphone part of this and try and improve the RF shielding.

73 Hugh M0WYE

Friday, 5 June 2020

Sealed Lead-acid Battery Charge Circuit for Jump Starter

On the left is a "Jump Starter" which contains a sealed lead-acid battery and originally had two big cables with crocodile clips to help start car engines when the car battery is flat. I never used it for that, I took off the big leads and added a fused cable to power my FT847 radio for portable operation. Not only does the case make the battery safe and easy to carry, but it also has a built in volt meter and an emergency lamp. On the right hand side there is a "cigarette lighter" type accessory socket to power other items or smaller radios.

The Jump Starter came with a mains adaptor to charge the battery via a power socket underneath the meter.

This is the weak-point of the system, because the adaptor is unregulated and when the battery is fully charged, the voltage reaches over 20 Volts. This has almost certainly caused the premature failure of the original battery, because lead-acid batteries should not be charged to a voltage higher than 14.4 Volts.
If the Voltage is regulated to 13.8V then the battery can be "float-charged" continuously.

There is plenty of room inside the box, and I have wanted to install a Voltage regulator for a long time. But I discovered that I could buy (very cheap!) a little regulator board from ebay.
The module came with a piddly little heatsink, but, as the regulator is close to the edge of the board, it is easy to add extra aluminium.

There are just four connections: input +/- (on the left) and output +/- (on the right). The voltage can be adjusted with the multi-turn preset at the bottom of the picture. It is wise to twiddle the adjustment screw to get 13.8 Volts before connecting the battery.

While adding extra heatsinking, I also added a smear of heatsink compound - as there was none on the device.

Now purists will complain that there should really be a current limit incorporated into the charge circuit, because a very discharged battery will draw a high current when charged. And this is true, however the existing circuit includes a one-ohm, high power resistor in series, and also the mains adaptor is rated at 500mA, and simply can't supply too much current for a lead-acid battery of this size. Whether the mains adaptor is really adequate is another matter.

Here is the circuit of the Jump Starter. The fused cable to the radio is connected straight across the battery terminals.

Hopefully the replacement battery will last longer now, although it is a smaller capacity than the original, which was claimed to be 12Ah. I say "claimed to be" as it weighed less than the 7Ah battery that I replaced it with and therefore probably had less lead in it!

Now the meter reads 13.8 Volts when the battery is charging.
This is good.

There is a data sheet for the LM317 regulator IC here:

The internals are a spaghetti of wires, but it is all well insulated :-).

The LM317 with a piece of scrap aluminium as a heatsink, board is mounted top right.

If you want to be scientific, then the "waste heat" that the regulator has to get rid of is the difference between the input and output voltage multiplied by the current flowing through the regulator. Commercial heatsinks are specified with "degrees per watt" which tells you the amount that the temperature of the heatsink will rise when it is dissipating 1 Watt and can be multiplied by the wasted power to work out whether you can fry eggs on it.

Or you can leave it on for a bit and feel (carefully) whether it has got warm or not. In my case not.
Hugh M0WYE

Thursday, 4 June 2020

PAM8403 Amplifier

I have some old audio-visual speakers, that were once used on a stand at exhibitions. They are quite battered, but the little 3" drive units seem ok. I thought I might make a pair of book-shelf speakers using them - sort of "PC speakers" that are a bit better quality than my old Sony ones.
I needed a little amp to drive them and spotted various amplifier modules on Ebay and Amazon. The speakers are rated at 3W so this PAM8403 module, at 3W per channel, seemed very suitable. PAM8403 Module from Ebay.
The module is tiny. It runs off 5V and has a stereo volume control with an on/off switch. There are input, output and power connections and that's it.
I hooked it up to a bench power supply, and a pair of the A-V speakers. I connected the headphone jack of my tablet to the input and was straight away listening to music at good volume level.
With everything turned up max, the music was very loud, but not distorting, so I am not sure that I had enough drive level to get the the full 3W. There was about 200mV going in when I looked on the scope.

Of course when I moved the scope to the output all I saw was a big blur! This is a "Class D" amplifier, that works using Pulse Width Modulation.
This is what I see on the speaker output with no signal going in. Here the timebase is set to 1uS per division, and the vertical scale is 2V/division. So this waveform is at 240kHz and is 8V peak to peak.

I guess this is going to radiate like crazy, and if I use this amplifier it may be best to use one module in each speaker and keep the speaker leads really short. Lots of filtering on the power-supply too.

The quiescent current was about 20mA and even when the volume was at maximum it was only drawing about 60mA. So it is incredibly efficient - 90% efficiency if the data is to be believed.

I found the datasheet for the chip here:

Anyway the amp module looks great for testing out the speakers. I can decide whether to carry on using it later.
Hugh M0WYE